Studies performed over the past decade give evidence that for lung cancer, increased radiotherapy dose will lead to better local control and survival. Further studies show that the main lung cancer radiotherapy side effect, pneumonitis, can be significantly reduced by lowering the dose delivered to healthy lung tissue. Balancing these conflicting goals is difficult, especially since a lung tumor moves on average 1.5-2 cm with respiratory motion. This project aims to develop and implement a technique entitled 4D IMRT (4-Dimensional Intensity Modulated Radiation Therapy). The 4th dimension, or time, component of the technique means that the tumor motion is accounted for during the imaging, planning and delivery of radiation. Tracking the tumor with the radiation beam during treatment delivery allows a reduction in beam margin and consequently a reduction in the volume of healthy tissue exposed to a high dose. The IMRT component of the technique allows us to give a sufficiently high dose to the tumor and an acceptably low dose to the healthy lung tissue. We will investigate the hypotheses that 4D IMRT will lead to: 1. improved treatment plans compared with conventional IMRT, 2. reduced volumes of normal tissue exposed to injurious doses compared with conventional IMRT, 3. reduced delivery time compared with respiration-gated IMRT, 4. wider applicability than breath-hold techniques, and 5. higher accuracy in delivered dose distributions compared with conventional and respiration-gated techniques.Though the studies here will concentrate mainly on lung cancer, 4D IMRT is applicable to all sites affected by respiratory motion, including breast and liver cancer, and even cervix and prostate cancer.